Force Information From A Trustee

The manager and trustee of my sisters trust account, set up by my late mother, refuses to provide any records of the management of the trust now that it has ended. How can I compel him to show us where the money went?

Additionally, he listed himself as 'co-trustee' when providing proof of closure to the state of New York so my sister could apply for social services, and I found an email from years ago from the family lawyer that I was actually a co-trustee.

Is it legal for this fella, a CPA with a kind of shady history in our dealings, to have acted independently all those years--assuming we were co-trustees--and it is legal for him to refuse all communication and requests for information now?

I live across the country or it would be easier to encourage him to communicate, and I can't think of a good reason he wouldn't reply to emails or phone calls. The last time I spoke to him by phone he was astonishingly rude and dismissive, much as he has always been on the topic of my sister, but simply being a rotten human being doesn't make him guilty of financial mismanagement. I just want the records, as he dispersed a very sizable amount of money.

Thanks for any advice on this.